The correct option that expresses the given sentence in passive voice is The ambulance was arranged for by his friends and he was immediately taken to the hospital.

The other options are incorrect because:

• His friends would have arranged for the ambulance and taken him immediately to the hospital is in the conditional tense, while the original sentence is in the simple past tense.
• His friends were arranged for the ambulance and taken him immediately to the hospital is grammatically incorrect. The verb "to arrange" is transitive, which means that it takes a direct object. However, the subject of the sentence is "his friends," which is not a direct object.
• The ambulance arranged for his friends and took him immediately to the hospital is grammatically incorrect. The subject of the sentence is "the ambulance," which is not the one who arranged for the ambulance.